The present invention relates to an output driving device used in a semiconductor memory device, and more particularly, to an output driving device which is capable of improving a slew rate.
Push-pull type drivers are widely used in buffers and driving amplifiers of semiconductor memory devices. In the push-pull type drivers, the control of the slew rate is very important.
A slew rate is defined as a maximum change rate of an output voltage per unit time. For example, in the case of an output driving device with a gain of 1, an input voltage immediately rises from 0 V to 1 V in an ideal circuit. However, in a practical circuit, when a slew rate is K, an output voltage does not rise simultaneously in response to an input voltage, but rises up to 1 V along a slope function with a slope of K. Therefore, controlling a slew rate of the output driving circuit to comply with a product specification is very important.
FIG. 1 is a circuit diagram of an output driving device of a typical semiconductor memory device.
Referring to FIG. 1, the output driving device includes a pull-up PMOS transistor 101UP and a pull-down NMOS transistor 101DN.
In the output driving device, a signal inputted through an input line 102 is inverted by the push-pull driving circuit, implemented with the pull-up PMOS transistor 101UP and the pull-down NMOS transistor 101DN, and outputted through an output line 103.
FIGS. 2 and 3 are waveform diagrams illustrating an output signal of the output driving device shown in FIG. 1.
FIG. 2 is a voltage waveform illustrating a voltage level of the output signal outputted at the output line 103 when the signal inputted through the input line 102 changes from a high state to a low state, where the output signal changes from a low state to a high state as the output signal corresponds to the inverted input signal. In this case, if the size of the PMOS transistor 101UP is increased, the signal outputted through the output line 103 rapidly changes from the low state to the high state.
FIG. 3 is a voltage waveform illustrating the output signal outputted at the output line 103 when the signal inputted through the input line 102 changes from a high state to a low state, where the output signal changes from a low state to a high state as the output signal corresponds to the inverted input signal. In this case, if the size of the NMOS transistor 101DN is increased, the signal outputted through the output line 103 rapidly changes from the low state to the high state.
As shown above, the slew rate is increased as the sizes of the pull-up PMOS transistor 101UP and the pull-down NMOS transistor 101DN are increased.
FIG. 4 is a structural diagram of an output driving device for controlling a slew rate.
Referring to FIG. 4, the output driving device includes a pull-up driving unit 401UP and a pull-down driving unit 401DN. The pull-up driving unit 401UP and the pull-down driving unit 401DN include a plurality of transistors for the slew rate control.
Specifically, the pull-up driving unit 401UP includes an upper PMOS transistor group provided between a power supply voltage terminal VDD and an output line 403 to receive signals PCODE<0> and PCODE<1> as input signals, and a lower PMOS transistor group configured to receive a signal through an input line 402.
The upper PMOS transistor group includes a plurality of PMOS transistors, whose sources are connected to the power supply voltage terminal VDD, receiving the signals PCODE<0> and PCODE<1> through their gates. The number of the PMOS transistors included in the upper PMOS transistor group is determined by the number of the PCODE signals. The lower PMOS transistor group includes a plurality of PMOS transistors having sources connected to drains of the PMOS transistors in the upper PMOS transistor group, drains connected to the output line 403, and gates receiving the signal inputted through the input line 402. The pull-up driving unit 401UP further includes a PMOS transistor having a source connected to the power supply voltage terminal VDD, a drain connected to the output line 403, and a gate connected to the input line 402.
By including the plurality of the PMOS transistors, the pull-up driving unit 401UP is possible to obtain an effect equivalent to controlling the size of the pull-up transistor 101UP illustrated in FIG. 1.
Likewise, the pull-down driving unit 401DN is implemented with a plurality of NMOS transistors.
That is, the pull-down driving unit 401DN includes a lower NMOS transistor group provided between a ground voltage terminal VSS and the output line 403 to receive signals NCODE<0> and NCODE<1> as input signals, and an upper NMOS transistor group configured to receive a signal through an input line 402.
The upper NMOS transistor group includes a plurality of NMOS transistors having drains connected to the output line 403 and gates receiving the signal inputted through the input line 402. The lower NMOS transistor group includes a plurality of NMOS transistors, whose drains are connected to sources of the NMOS transistors in the upper NMOS transistor group and sources are connected to the ground voltage terminal VSS, receiving the signals NCODE<0> and NCODE<1> through their gates. The number of the NMOS transistors included in the lower NMOS transistor group is determined by the number of the NCODE signals.
The pull-down driving unit 401DN further includes an NMOS transistor having a drain connected to the output line 403, a source connected to the ground voltage terminal VSS, and a gate connected to the input line 402.
By including the plurality of the NMOS transistors, the pull-down driving unit 401DN is possible to obtain an effect equivalent to controlling the size of the pull-down transistor 101DN illustrated in FIG. 1 by using the signals PCODE<0> and PCODE<1>.
Therefore, the output driving device shown in FIG. 4 is configured to obtain the effect equivalent to controlling the sizes of the pull-up PMOS transistor 101UP and the pull-down NMOS transistor 101DN illustrated in FIG. 1.
That is, the pull-up driving unit 401UP operates the PMOS transistors included in the upper PMOS transistor group in response to the signals PCODE for controlling the slew rate according to the specification of the product. Also, the PMOS transistor in the lower PMOS transistor group are operated in response to the signal inputted through the input line 402 to output an output signal through the output line 403. Furthermore, the pull-down driving unit 401DN operates the NMOS transistors in the lower NMOS transistor group in response to the signals NCODE for controlling the slew rate according to the specification of the product. Also, the NMOS transistors in the upper NMOS transistor group are operated in response to the signal inputted through the input line 402 to output the output signal through the output line 403.
However, the driving power of the output driving device shown in FIG. 4 is inefficient because the driving power is set based upon the largest transistor in order to drive the PMOS transistors and the NMOS transistors having various sizes, which constitute the output driving device. Consequently, the conventional output driving device has the above limitation since the slew rate is controlled by using the size of the transistors.